1. Technical Field
The present invention relates to an illumination device and a projector.
2. Related Art
As an illumination device for a projector, there have been known an illumination device described in JP-A-2009-277516 (Document 1) and an illumination device described in JP-A-2010-164846 (Document 2). In either of the illumination devices, the fluorescent body is excited by excitation light emitted from the light source to thereby emit fluorescence from the fluorescent body. In the illumination device of Document 1, a fluorescent layer is provided on a rotatable substrate (a fluorescent wheel). In the illumination device of Document 2, a fan-shaped fluorescent body for emitting red fluorescence, a fan-shaped fluorescent body for emitting blue fluorescence, and a fan-shaped fluorescent body for emitting green fluorescence are disposed on a rotatable fluorescent wheel so as to form a circle. In either of the illumination devices, the lights used as the image light and having respective colors different from each other can sequentially be obtained by irradiating the fluorescent wheel with the laser beam emitted from the light source while rotating the fluorescent wheel in a circumferential direction.
When raising the output level of the laser beam which is emitted from the light source and is incident on the fluorescent body from, the luminance of the fluorescence emitted by the fluorescent body rises while the temperature of the part of the fluorescent body thus irradiated becomes high to thereby accelerate deterioration of the fluorescent body. However, since the fluorescent layer is disposed on the rotatable substrate in either of the illumination devices, even if the part of the fluorescent layer corresponding to a focused spot is irradiated with the excitation light to thereby heat the fluorescent layer, the part promptly gets out of the focused spot, and is cooled by the ambient air until when the part is irradiated with the excitation light again. Therefore, rise in temperature of the fluorescent layer is suppressed, and deterioration in the fluorescent layer and degradation of wavelength conversion efficiency of the fluorescent layer are hard to occur compared to the case in which a certain part is continuously irradiated with the excitation light. Further, in the illumination device of Document 2, the rotating fluorescent wheel is reciprocated in the radial direction by operating a crank mechanism. Thus, the position on the fluorescent body where the laser beam is incident on is shifted in the radial direction of the fluorescent wheel to thereby further reduce the thermal damage of the fluorescent body.
Since the substrate of the illumination device of Document 1 rotates, the part irradiated with the excitation light in a certain round is cooled by the ambient air until the part is irradiated with the excitation light in the subsequent round again (after one rotational period elapses). However, the area in the fluorescent layer irradiated with the excitation light in each of the rounds is the same. In other words, since a certain part of the fluorescent layer is repeatedly irradiated with the excitation light in every round at short time intervals, the part is not sufficiently cooled, and it becomes unachievable to sufficiently suppress the rise in temperature of the fluorescent layer. As a result, there arise the deterioration of the fluorescent layer and the degradation of the wavelength conversion efficiency of the fluorescent layer.
In the illumination device of Document 2, a second electric motor for operating the crank mechanism is required besides a first electric motor for rotating the fluorescent wheel. Therefore, the structure thereof is complicated, which poses a problem for downsizing of the projector.